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Insect Cuticle• Dynamic, not inert• Functions as skin AND

skeleton• Strong but transmits

information and substances

• Gives shape, color, pattern

Outline• Shape –macro and microstructure• General cuticle structure: chitin, protein• Factors changing cuticle properties:

sclerotization, water content• Resilin - ‘cuticle’ without chitin• Outer surface and coatings for special

properties: waxes, color• Cuticle replacement by molting

SHAPE - the cuticle is not a flat sheet

• major features• microfeatures

scaffold for strength, muscle attachments

• pleural (suture) (example)• critical for flight

Sculpture at many levels

• specialized hair and socket cells

• one structure per cell - cells unspecialized

• multiple structures per cell

• multiple cells

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Two major sections:

1. epicuticle2. procuticle

epidermal cells

Functions of the two sections

• The epicuticle and coatings made by the cells give surface properties such as waterproofing

• The procuticle, with its composite structure, provides mechanical properties such as stiffness and elasticity

• Epicuticle –made up of cuticulin and inner epicuticle

epicuticlecuticulin is produced at

plasma membrane surface

Mostly highly polymerized lipid

cuticulin

cuticulin

epicuticleformed by release of material from vesicles that assemble under envelope

epidermis and associated cells

• epidermal cells• glandular cells• oenocytes

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Cellular layer• epidermal cells make new cuticle• associated cells. For example,

oenocytes produce hydrocarbons, lipids, and wax (icing on cuticle)

Procuticle

• What is it made of?• How is it put together?• How do the components vary to give

such a wide range of properties?

Cuticle is a composite material

• CHITIN fibers• PROTEINS matrix

Basic unit of chitin is n-acetylglucosamine

β - linkage Chitin makes up as much as half of the exoskeleton

forms long chains

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chains interact with each otherhydrogen bondsform microfibrils

Helical pattern of layers

Means that strength is same in all directions

Chitin fibrils form layers

Chitin

• n-acetylglucosamine units

• form chains

• form microfibrils

• form layers

Composite Materials

• versatile, light, different properties based on different combinations

fibers stacked layersmatrix of proteins and other component

• Growing field of materials engineering and design that is “bioderived and bioinspired”

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Sclerotization

• hardening of the cuticle by chemical interactions among components

Across insect cuticle, sclerotization varies

• Exocuticle = hardened region

• Endocuticle= not hardened

Exocuticle

Endocuticle

Degree of sclerotization varies in different body parts, stages,

species … etcRegions of unsclerotized cuticle give points/lines that can bend

Proteins are key to diverse mechanical properties

• interactions of protein with chitin• interactions of protein with protein• water content and pH change how proteins

interact

a cuticle protein

• “cleft” full of aromatic residues, which form “flat”surfaces of aromatic rings, for protein–chitin interactions

• outer surface (lower side) important for protein–protein interactions in cuticle.

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How can proteins contribute to different cuticle properties

(hard or soft)?

Making hard cuticle

A protein in hard cuticle

• Histidines (blue) are in right position to participate in sclerotization

• Or to be involved in water binding capacity of cuticle

n-acetyldopamine quinoneis common in sclerotized

(hard) cuticle A protein in soft cuticle

• lacks histidinesfor sclerotization

additional hardening with metal

• e.g., zinc in mandibles and ovipositor of a wasp

Water

• Hard, stiff cuticles contain 15-35% chitin and only 12 % water

• Soft cuticle contains equal parts chitin and protein AND 40-75% water

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Effect of water content on shear stiffness

• very small % increase in water makes huge difference in stiffness

Young’s modulus

= stiffness

insect cuticle shows a huge range of stiffness across a very narrow range of density

Some important factors are:•Quinones •Proteins and protein structure•Metal•Water content

In some cases, properties can change reversibly

Plasticization

• Rhodnius• cuticle only 10% chitin • increases water

content from 26 to 31% and increases its extensibility from about 10% to 100%

Plasticization

• controlled by hormones

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Resilin

• contains NO chitin• rubber-like protein • stores energy• small bits are important in many insects

body parts

flea • Resilin in flea leg and

internal supports is a key element in building up energy for a jump

Dermaptera• very important in

wing flexibility and resilience

• blue areas contain resilin

Resilin cloned• Resilin gene cloned

into E. coli• Product isolated• Cross linked

photochemically• Resilience is better

than man-made high resilience rubber.

• Great potential in biomedical applications

engineers at workFunctions of the two sections

• The epicuticle and coatings, made by the cells, give surface properties

• The procuticle, with its composite structure, provides mechanical properties such as stiffness and elasticity

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Pore Canals

epidermal cells have extensions that reach up through the epicuticle

wax decorations

• water barrier• reflection• camouflage• ?other

Color

• Pigments• Structural colors

Some Pigments

• Pterins - yellow, red, white

• Ommochromes -yellow, red, brown

• Quinones - Homoptera only

Structural colors

• Entomologists don’t do optics, physicists don’t do biology

• Entomological vocabulary has about 30 terms to distinguish shades of brown, but only one for iridescence

Seago et al. 2009. Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles. J. R. Soc. Interface 6, S165-S184.

3 main classes of iridescence(color changes with angle)

• multilayer reflectors• diffraction gratings• photonic crystals (opalescent)

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Multilayer reflectors

Seago A E et al. J. R. Soc. Interface 2009;6:S165-S184

©2009 by The Royal Society

Tiger beetles

• unlayeredepicuticleof a black beetle

• layered epicuticle of an iridescent red beetle

• layer spacing has peak green reflectance

Seago A E et al. J. R. Soc. Interface 2009;6:S165-S184

©2009 by The Royal Society

“Additive” coloration • pointilistic disruption of even color• another way to reduce iridescence

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Seago A E et al. J. R. Soc. Interface 2009;6:S165-S184

©2009 by The Royal Society

Circularly polarized multilayer reflectors

one rotation=wavelength of lightanalogous to cholesteric liquid crystal

rare, only in scarabs

Broadband multilayer reflectors

Seago A E et al. J. R. Soc. Interface 2009;6:S165-S184

©2009 by The Royal Society

• the broader the range of thicknesses, the closer to pure silver or gold

Multilayer reflectors

• simple layered reflectors• additive color mixing (pointilistic)• circular polarizing reflectors• broad band reflectors

Physical color by diffraction Butterfly scales

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The iridescent scales of the Morpho sulkowskyi butterfly give a different optical response to different individual vapours. This optical response dramatically outperforms that of existing nano-engineered photonic sensors.

And every molt they make a new one!

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What happens if these cells make new cuticle?

• It will be the same size as the one before

• FIRST, cell division!

new cuticle will form on top of this larger epidermis

APOLYSIS-separation of old cuticle

from epidermis, formation of space

• Molting fluid

• New cuticulinand epicuticle

Enzymes activatedInner epicuticle produced

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Endocuticledigested

Fluid reabsorbed

Procuticle laid down

Procuticle deposition a 2 stage process

• chitin and specific proteins that coat it • then other proteins

pharate pupal stage inside larval cuticle

ecdysis Expansion, sclerotization